In sheet-fed printing machines, the sheets to be printed are removed from the top of a feeder stack, conveyed over a conveyor table to a feeder rest and aligned on a feeder lay. Sheets properly resting on the feeder lay are gripped by a pre-gripper and conveyed into the first printing unit where printing commences. The starting-up/restarting of continuous printing requires various steps to properly initiate this printing process. Procedurally, the process of starting-up/restarting of continuous printing is performed by cutting-in a feeder assembly at a basic rotational speed and simultaneously cutting-in sheet removal actuators. The basic rotational speed is illustratively 5,000 sheets per hour. Cutting-in is achieved by activating switchable clutches coupled to the drive of the printing machine which initiate the removal and conveyance of sheets.
The printing machine executes a number of revolutions before the first sheet reaches the feeder rest since several sheets rest on the conveyor table at one time. As sheets are conveyed on the conveyor table between the feeder stack and the first printing unit in an imbricated fashion, they travel at a speed slower than the speed of the sheets traveling within the printing machine. Once in the printing unit, the sheets are accelerated to the continuous printing speed.
During the run-up operation a certain number of reject sheets are produced. Rejects occur for various reasons including inadequate ink saturation in the first sheets printed due to the fact that the ink/damping solution equilibrium is not established prior to the first sheets entering the printing unit. Rejects also occur on account of drive torsion and mackling which results when sheets are accelerated to the continuous printing speed from a resting position after waiting for the print machine to complete the run-up process.
Disclosed in U.S. Pat. No. 5,584,244 (and corresponding German Patent DE 4 407 631 C1) is a method for starting the production run on a sheet-processing printing machine. According to that method, the feeder assembly is cut-in at a basic rotational speed while the sheet removal actuators are disengaged. After the printing machine is run-up to the continuous printing speed, the paper supply is cut-in causing removal of sheets from the stack and conveyance into the printing machine. This method reduces the large number of reject sheets incurred during the start-up process by ensuring that a first sheet entering the printing machine for printing is printed at the continuous printing speed.
Disclosed in U.S. Pat. No. 5,626,077 (and corresponding German Patent DE 195 05 560 A1) is a method for controlling sheet feed. According to that method, a controller is utilized to determine the presence of double or misfed sheets as the sheets are removed from a sheet stack in a feeder unit. Upon the detection of a double or misfed sheet, the controller, through actuating devices in the machine, shuts down various printing machine functions such that the sheets ahead of the double or misfed sheet complete the printing process and the machine ceases operating precisely at the point where the double or misfed sheet reaches the feeder rest. This method avoids the situation where sheets located on the conveyor table are discarded after detection of a double or misfed sheet. This method also turns off the ink feed and damping solution feed such that the sheets still printing, after detection of a double or misfed sheet, pick up the ink from the rubber blanket and printing plate. Thus, over-inking of the first sheet is avoided when printing resumes.
Disclosed in DE 195 10 082 C2 (corresponding to U.S. application Ser. No. 08/618,786, which is assigned to the same assignee as that of the present application) is a method and apparatus for controlling the sheet supply in a sheet processing printing machine. According to that method, sheets are conveyed across a conveyor table to a feeder rest in a printing unit. When the first sheet hits the predetermined position in the printing machine, the sheet feed process is stopped and a process implementing predamping and preinking in a first printing unit is started. After the predamping and preinking process is complete, the sheet feed process is restarted and a first sheet enters the printing zone. This method also reduces reject sheets by ensuring that a first sheet entering the printing machine for printing receives the optimal ink and is not over or under saturated. A disadvantage associated with this method is that the sheet feed process is halted while waiting for the preinking and predamping process to complete.